This invention relates to a sheet detection apparatus for detecting a sheet loaded on a printer or typewriter, more particularly, to a sheet detection apparatus using a sensor of light reflection types or a combination of sensor of light reflection type and a transmission type sensor for detecting various types of sheets.
There has been known a sheet detection apparatus using a sensor of light reflection type or a sensor of light transmission type for detecting whether a sheet is present or absent.
In paper detection apparatus which has been known, as shown in FIG. 1, a sensor of light reflection type 91 comprising a light emission element and a light reception element is provided adjacent to a print head 94 on a carriage 95 transversely traveled along a platen 93, as the print head 94 travels, the amount of reflected light being detected, thereby determining whether a paper is present or absent and where it is. In the structure, the platen 93 is positioned on an opposite side of the sensor of light reflection type 91 and the surface of the platen 93 is dark so as to reduce the amount of reflected light when a paper 99 is absent. Namely, platens with dark rubber or aluminum whose surface is painted or coated in dark have been used.
In the structure described above, when the white paper 99 is fed to the platen 93, the carriage 95 which is provided with the sensor of light reflection type 91 travels. When the sensor of light reflection type 91 is located at a position opposed to the white paper 99, the sensor of light reflection type 91 inputs much light compared with a state where the white paper 99 is absent, thereby detecting that the white paper 99 is present.
However, in the paper detection apparatus described above, several practical problems have been pointed out. For example, when the apparatus is used for a long time and the surface of the platen 93 is dirtied by ink or the dark paint applied on the platen 93 is peeled off, the sensor of light reflection type 91 cannot input proper amount of reflected light, whereby the white paper 99 may not be occasionally detected.
In addition, to detect the white paper 99 using reflected light, the color of the surface opposed to the sensor of light reflection type 91 should be dark. Since the surface of the platen 93 has been usually dark, presence or absence of a dark paper or high tone color paper and its position could not have been precisely detected. Moreover, when detecting a transparent film used for overhead projection, it has been impossible to determine whether the transparent film is present or absent and where it is by detecting a change of reflected light like the situation of the dark paper.
Meanwhile, in recording apparatus which has been recently announced, as multiple color printing technologies have advanced, a variety of printing sheet types have been used such as high tone color sheets and dark sheets besides white sheets.
However, in conventional sheet detection apparatus described above, when a variety of sheet types are detected, errors occur because the light reflection factor and light transmission factor depend on the sheet type to be used.
Although it is possible to consider using a mechanical contact type switch which does not use light to detect a sheet, the contacts of the switch have a life restricted by operation time. Therefore, when the contacts do not work due to a long time use, the sheet cannot be detected.